The present invention relates to a non-aqueous electrolytic solution secondary battery, and in particular relates to a non-aqueous electrolytic solution secondary battery where an electrode group having a positive electrode, a negative electrode and a separator, connecting portions which connect to respective terminals from the electrode group, and a non-aqueous electrolytic solution are accommodated in a battery container provided with an internal pressure releasing mechanism which releases internal pressure at a predetermined pressure and where the positive electrode is constituted by applying a positive electrode active material mixture including lithium-manganese complex oxide and conductive material on both surfaces of a foil-shaped positive electrode collector, and the negative electrode is constituted by applying a negative electrode active material mixture including carbon material on both surfaces of a foil-shaped negative electrode collector.
Because a non-aqueous electrolytic solution secondary battery represented by a lithium-ion secondary battery has a high energy density as its merit, it is mainly used as a power source or power supply for portable equipment such as a VTR camera, a notebook type personal computer, a portable or cellar telephone or the like. The interior structure of this battery is generally of a winding type as described below. Each of a positive electrode and a negative electrode of the battery is formed in a strip-shape where active material is applied to a metal foil, and a winding group is spirally formed by winding the positive electrode and the negative electrode through a separator so as not to come in direct contact with each other. This winding group is accommodated in a cylindrical battery container or can, and, after the battery container is filled with electrolytic solution, it is sealed.
An ordinary cylindrical lithium-ion secondary battery has an external dimension of a diameter of 18 mm and a height of 65 mm, which is called 18650 type, and it is widely spread as a small-sized non-aqueous electrolytic solution secondary battery for a civilian use. Lithium cobaltate having a high capacity and a long life is mainly used as a positive electrode active material for the 18650 type lithium-ion secondary battery, and battery capacity of the 18650 type lithium-ion secondary battery is approximately 1.3 Ah to 1.7 Ah and battery power (output) is about 10 W or so.
Meanwhile, in order to cope with the environmental problems in the automotive industry, development of electric vehicle (EVs) whose power sources are confined completely to batteries so that there is no gas exhausting and development of hybrid electric vehicles (HEVs) where both internal combustion engines and batteries are used as their power sources have been facilitated and some of them have reached a practical state. Such a battery which is a power source for EV or HEV is required to have high power and high energy characteristics, and an attention is being paid to a lithium-ion battery as a battery which meets such requirements.
In order to spread these EVs and HEVs, it is essential to reduce the cost of such a battery. For this reason, it is required to use low-cost battery materials, where, in a case of a positive electrode active material, for example, a special attention is being paid to manganese oxides which are rich as natural resources and improvement of such batteries has been conducted for high performance thereof. Also, as the batteries for the EVs and HEVs, not only high capacity but also high power which affects acceleration of a vehicle, namely reduction of the internal resistance of the battery, are required. In order to increase the reaction area of the electrode, this requirement can be met by utilizing a lithium manganate having a large specific surface area as the positive electrode active material.
However, in a case of the lithium-ion battery, according to an increase in capacity and power, the safety is apt to lower. Particularly, as mentioned above, in the case that lithium manganate aiming at high power is used, such a tendency appears that a phenomenon of the battery becomes violent when it falls in an abnormal state. In a battery having high capacity and high power such as used for a power source for EV or HEV, since large current charging and large current discharging are performed, it is substantially difficult to provide within the battery a current shutting-off mechanism (a kind of a cutting-off switch) which actuates according to an increase in internal pressure at an abnormal time, such as employed in the 18650 type lithium-ion battery.
Also, in a case in which a large-sized non-aqueous electrolytic solution secondary battery is used as, for example, a power source for EV or HEV, safety must always be secured even at an abnormal time such as (1) at a time of overcharging due to failure in a charging control system, (2) at a time of crushing due to an accidental collision, (3) at a time of foreign matter spitting, (4) at a time of external short-circuiting or the like. That is, it is an important problem that behavior of the battery, when it has fallen into an abnormal state (at the abnormal time), does not injure a person or passenger and damage to a vehicle is suppressed to a minimum.
In view of the above circumstances, a first object of the present invention is to provide a non-aqueous electrolytic solution secondary battery which has high safety while maintaining high capacity and high power. Also, a second object of the present invention is to provide a non-aqueous electrolytic solution secondary battery which can secure safety even at an abnormal time of the battery.
In order to achieve the first object, according to a first aspect of the present invention, there is provided a non-aqueous electrolytic solution secondary battery where an electrode group having a positive electrode, a negative electrode and a separator, connecting portions which connect to respective terminals from the electrode group, and a non-aqueous electrolytic solution are accommodated in a battery container provided with an internal pressure releasing mechanism which releases internal pressure at a predetermined pressure and where the positive electrode is constituted by applying a positive electrode active material mixture including lithium-manganese complex oxide and conductive material on both surfaces of a foil-shaped positive electrode collector, and the negative electrode is constituted by applying a negative electrode active material mixture including carbon material on both surfaces of a foil-shaped negative electrode collector, wherein the lithium-manganese complex oxide is set such that an amount of elution of manganese into the non-aqueous electrolytic solution is 5% or less on the basis of the lithium-manganese complex oxide in a range where an electrode potential to metal lithium is 4.8 V or more, and the carbon material is graphite in/from which lithium ions can be occluded/released according to charging/discharging.
Also, in order to the first object, according to a second aspect of the present invention, there is provided a non-aqueous electrolytic solution secondary battery where an electrode group having a positive electrode, a negative electrode and a separator, connecting portions which connect to respective terminals from the electrode group, and a non-aqueous electrolytic solution are accommodated in a battery container provided with an internal pressure releasing mechanism which releases internal pressure at a predetermined pressure and where the positive electrode is constituted by applying a positive electrode active material mixture including lithium-manganese complex oxide and conductive material on both surfaces of a foil-shaped positive electrode collector, and the negative electrode is constituted by applying a negative electrode active material mixture including carbon material on both surfaces of a foil-shaped negative electrode collector, wherein the lithium-manganese complex oxide is set such that an amount of elution of manganese into the non-aqueous electrolytic solution is 7% or less on the basis of the lithium-manganese complex oxide in a range where an electrode potential to metal lithium is 4.8V or more, and the carbon material is amorphous carbon in/from which lithium ions can be occluded/released according to charging/discharging.
In the present invention, in order to secure a non-aqueous electrolytic solution secondary battery having high capacity and high power (output), the lithium-manganese complex oxide is used as the active electrode active material and the carbon material is used as the negative electrode active material. In the non-aqueous electrolytic solution secondary battery with high capacity and high power, when it has fallen into an abnormal state, large current charging or large current discharging is maintained and a large amount of gas is generated urgently due to chemical reaction between the non-aqueous electrolytic solution and the active material mixture within a battery container so that internal pressure in the battery container is increased. This tendency is calmer in the lithium-manganese complex oxide than a cobalt-manganese complex oxide or a nickel-manganese complex oxide. In general, in a non-aqueous electrolytic solution secondary battery, in order to prevent the internal pressure in a battery container from increasing, an internal pressure releasing mechanism for releasing the internal pressure at a predetermined pressure is provided in the battery container. In the above aspects of the present invention, however, such a configuration that the lithium-manganese complex oxide which is set such that an amount of elution of manganese into the non-aqueous electrolytic solution is 5% or less on the basis of the lithium-manganese complex oxide in a range where an electrode potential to metal lithium is 4.8V or more is used as the positive electrode active material and that the carbon material which is graphite in/from which lithium ions can be occluded/released according to charging/discharging is used as the negative electrode active material, or such a configuration that the lithium-manganese complex oxide which is set such that an amount of elution of manganese into the non-aqueous electrolytic solution is 7% or less on the basis of the lithium-manganese complex oxide in a range where an electrode potential to metal lithium is 4.8V or more is used as the positive electrode active material and that the carbon material is amorphous carbon in/from which lithium ions can be occluded/released according to charging/discharging is used as the negative electrode active material is employed, gas discharging form the internal pressure releasing mechanism can be performed remarkably gently. For this reason, according to the present invention, a non-aqueous electrolytic solution secondary battery which has considerably high safety while maintaining high capacity and high power can be realized.
In the above aspects, when a Li/Mn composition ratio in the lithium-manganese complex oxide is set in a range of form 0.55 to 0.6, a non-aqueous electrolytic solution secondary battery which has a high level in an initial capacity and a capacity retaining percentage due to charging/discharging can be manufactured. At this time, in a case that amorphous carbon is used as the negative electrode active material, and the amount of elution of manganese of the lithium-manganese complex oxide into the non-aqueous electrolytic solution is set to 3.2% or less on the basis of the lithium-manganese complex oxide, even when the non-aqueous electrolytic solution secondary battery has fallen into the abnormal state, heat generation due to the chemical reaction between the non-aqueous electrolytic solution and the active material mixture can be suppressed to a low level, and safety and reliability of the non-aqueous electrolytic solution secondary battery can be improved more. From a viewpoint of material preparation and cost, it is preferable that lithium manganate among the lithium-manganese complex oxides is used as the positive electrode active material.
Moreover, lithium-manganese complex oxide where a half band width change of main diffraction light due to X-ray diffraction between SOC 0% and SOC 100% is set to 25% or less is used as the positive electrode active material, since a thermally stable structure can be obtained due that lowering of spinel crystallization in the lithium-manganese complex oxide is suppressed, manganese elution and oxygen discharge from the lithium-manganese complex oxide are suppressed at the abnormal time. Thus, by using such a specific lithium-manganese complex oxide as the positive electrode active material, the manganese elution and the oxygen discharge from the lithium-manganese complex oxide can be suppressed at the abnormal time, so that the amount of heat generation occurring according to decomposition reaction between manganese and oxygen can be decreased and the internal pressure can be released calmly from the internal pressure releasing mechanism. Accordingly, safety can be secured even at the abnormal time, and thereby the second object can be achieved.
At this time, when a Li/Mn composition ratio in the lithium-manganese complex oxide is set in a range of from 0.55 to 0.60, the amount of elution of manganese can be reduced more and heat generation is made difficult at the abnormal time as compared with a case of a stoichiometric composition (Li/Mn composition ratio=0.5), which results in preferable improvement in safety of the battery. Such a lithium-manganese complex oxide can include one where a portion of manganese thereof is substituted with another metal.